Method and apparatus for multi-user concurrent random access for wireless local area networks (wlans)

ABSTRACT

A method and apparatus for multi-user concurrent random access for wireless local area networks (WLANs) is described. The method, implemented in a wireless transmit/receive unit (WTRU), includes detecting a trigger frame for uplink (UL) multi-user (MU) transmission. The trigger frame includes an assignment of resource units (RUs) for random access in upcoming UL MU packet data convergence protocol (PDCP) protocol data units (PPDUs) and an indication that the trigger frame is one of a plurality of trigger frames in a cascading sequence of trigger frames in an MU transmission opportunity (TxOP). The method further includes selecting one of the RUs in the assignment of RUs for a random access transmission and sending the random access transmission on the selected one of the RUs.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/217,564, which was filed on Sep. 11, 2015, U.S.Provisional Patent Application No. 62/242,484, which was filed on Oct.16, 2015, and U.S. Provisional Patent Application No. 62/278,774, whichwas filed on Jan. 14, 2016, the contents of which are herebyincorporated by reference herein.

SUMMARY

A method and apparatus for multi-user concurrent random access forwireless local area networks (WLANs) is described. The method,implemented in a wireless transmit/receive unit (WTRU), includesdetecting a trigger frame for uplink (UL) multi-user (MU) transmission.The trigger frame includes an assignment of resource units (RUs) forrandom access in upcoming UL MU packet data convergence protocol (PDCP)protocol data units (PPDUs) and an indication that the trigger frame isone of a plurality of trigger frames in a cascading sequence of triggerframes in an MU transmission opportunity (TxOP). The method furtherincludes selecting one of the RUs in the assignment of RUs for a randomaccess transmission and sending the random access transmission on theselected one of the RUs.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawingswherein:

FIG. 1A is a system diagram of an example communications system in whichone or more disclosed embodiments may be implemented;

FIG. 1B is a system diagram of an example wireless transmit/receive unit(WTRU) that may be used within the communications system illustrated inFIG. 1A;

FIG. 1C is a system diagram of an example radio access network and anexample core network that may be used within the communications systemillustrated in FIG. 1A;

FIG. 2A is a diagram illustrating example building blocks for an example20 MHz orthogonal frequency-division multiple access (OFDMA) UL MU PPDUfor 20 MHz that may be sent in response to a trigger frame;

FIG. 2B is a diagram of an example high efficiency signal B (HE-SIG-B)field;

FIGS. 3A and 3B are diagrams of example trigger frames for MU randomaccess;

FIG. 4A is a system diagram illustrating a potential scenario that maycause a collision that may be remedied by a null data packet (NDP)-typetrigger;

FIG. 4B is a system diagram illustrating how use of the NDP-type triggermay prevent the collision in the scenario of FIG. 4B;

FIG. 5 is a diagram of an example short random access (SRA) mediumaccess control (MAC) frame;

FIG. 6A is a diagram of an example NPD SRA frame;

FIG. 6B is a diagram of another example NPD SRA frame;

FIG. 6C is a diagram of another example NDP SRA frame;

FIG. 7 is a diagram of an example semi-NDP SRA frame;

FIGS. 8A and 8B are signal diagrams of example frame exchanges withrandom access and trigger frames;

FIG. 9 is a flow diagram of an example method of triggering UL MU randomaccess;

FIG. 10 is a flow diagram of an example method of UL MU random access

FIG. 11 is a flow diagram of another example method of UL MU randomaccess;

FIG. 12 is a diagram of a random access procedure with SRA frames;

FIG. 13 is a signal diagram of an example sequential random accessprocedure using cascading trigger frames where random accessopportunities (RaOPs) occur after inter-spaced block acknowledgement(ACK)/trigger frames;

FIG. 14 is a signal diagram of an example sequential random accessprocedure using cascading trigger frames where RaOPs occur immediatelyafter each other with a delayed MU-Block ACK;

FIG. 15 is a signal diagram of an example PHY layer acknowledgementprocedure when delayed ACK is permitted;

FIG. 16 is a flow diagram of an example method of UL MU random access;

FIG. 17 is a flow diagram of another example method of UL MU randomaccess;

FIG. 18 is a diagram illustrating a potential collision that may occur aresult of the STA returning to the normal EDCA procedure after a UL MUrandom access procedure or scheduled UL transmission of a short packet;

FIG. 19 is a diagram of an example UL MU PPDU;

FIG. 20 is a graph of the power spectrum density of a partially loadedOFDM signal with radio frequency (RF) in-phase/quadrature (I/Q)imbalance;

FIG. 21 is a diagram showing an example of symmetric random access;

FIG. 22 is a signal diagram of an example random access continuoustransmission opportunity (TxOP) with traffic priorities determinedwithin each random access opportunity; and

FIG. 23 is a signal diagram of an example random access continuous TxOPwith traffic priorities determined for specific RUs.

DETAILED DESCRIPTION

FIG. 1A is a diagram of an example communications system 100 in whichone or more disclosed embodiments may be implemented. The communicationssystem 100 may be a multiple access system that provides content, suchas voice, data, video, messaging, broadcast, etc., to multiple wirelessusers. The communications system 100 may enable multiple wireless usersto access such content through the sharing of system resources,including wireless bandwidth. For example, the communications systems100 may employ one or more channel access methods, such as code divisionmultiple access (CDMA), time division multiple access (TDMA), frequencydivision multiple access (FDMA), orthogonal FDMA (OFDMA), single-carrierFDMA (SC-FDMA), and the like.

As shown in FIG. 1A, the communications system 100 may include wirelesstransmit/receive units (WTRUs) 102 a, 102 b, 102 c, 102 d, a radioaccess network (RAN) 104, a core network 106, a public switchedtelephone network (PSTN) 108, the Internet 110, and other networks 112,though it will be appreciated that the disclosed embodiments contemplateany number of WTRUs, base stations, networks, and/or network elements.Each of the WTRUs 102 a, 102 b, 102 c, 102 d may be any type of deviceconfigured to operate and/or communicate in a wireless environment. Byway of example, the WTRUs 102 a, 102 b, 102 c, 102 d may be configuredto transmit and/or receive wireless signals and may include userequipment (UE), a mobile station, a fixed or mobile subscriber unit, apager, a cellular telephone, a personal digital assistant (PDA), asmartphone, a laptop, a netbook, a personal computer, a wireless sensor,consumer electronics, and the like.

The communications systems 100 may also include a base station 114 a anda base station 114 b. Each of the base stations 114 a, 114 b may be anytype of device configured to wirelessly interface with at least one ofthe WTRUs 102 a, 102 b, 102 c, 102 d to facilitate access to one or morecommunication networks, such as the core network 106, the Internet 110,and/or the other networks 112. By way of example, the base stations 114a, 114 b may be a base transceiver station (BTS), a Node-B, an eNode B,a Home Node B, a Home eNode B, a site controller, an access point (AP),a wireless router, and the like. While the base stations 114 a, 114 bare each depicted as a single element, it will be appreciated that thebase stations 114 a, 114 b may include any number of interconnected basestations and/or network elements.

The base station 114 a may be part of the RAN 104, which may alsoinclude other base stations and/or network elements (not shown), such asa base station controller (BSC), a radio network controller (RNC), relaynodes, etc. The base station 114 a and/or the base station 114 b may beconfigured to transmit and/or receive wireless signals within aparticular geographic region, which may be referred to as a cell (notshown). The cell may further be divided into cell sectors. For example,the cell associated with the base station 114 a may be divided intothree sectors. Thus, in one embodiment, the base station 114 a mayinclude three transceivers, i.e., one for each sector of the cell. Inanother embodiment, the base station 114 a may employ multiple-inputmultiple-output (MIMO) technology and, therefore, may utilize multipletransceivers for each sector of the cell.

The base stations 114 a, 114 b may communicate with one or more of theWTRUs 102 a, 102 b, 102 c, 102 d over an air interface 116, which may beany suitable wireless communication link (e.g., radio frequency (RF),microwave, infrared (IR), ultraviolet (UV), visible light, etc.). Theair interface 116 may be established using any suitable radio accesstechnology (RAT).

More specifically, as noted above, the communications system 100 may bea multiple access system and may employ one or more channel accessschemes, such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA, and the like. Forexample, the base station 114 a in the RAN 104 and the WTRUs 102 a, 102b, 102 c may implement a radio technology such as Universal MobileTelecommunications System (UMTS) Terrestrial Radio Access (UTRA), whichmay establish the air interface 116 using wideband CDMA (WCDMA). WCDMAmay include communication protocols such as High-Speed Packet Access(HSPA) and/or Evolved HSPA (HSPA+). HSPA may include High-Speed DownlinkPacket Access (HSDPA) and/or High-Speed Uplink Packet Access (HSUPA).

In another embodiment, the base station 114 a and the WTRUs 102 a, 102b, 102 c may implement a radio technology such as Evolved UMTSTerrestrial Radio Access (E-UTRA), which may establish the air interface116 using Long Term Evolution (LTE) and/or LTE-Advanced (LTE-A).

In other embodiments, the base station 114 a and the WTRUs 102 a, 102 b,102 c may implement radio technologies such as IEEE 802.16 (i.e.,Worldwide Interoperability for Microwave Access (WiMAX)), CDMA2000,CDMA2000 1×, CDMA2000 EV-DO, Interim Standard 2000 (IS-2000), InterimStandard 95 (IS-95), Interim Standard 856 (IS-856), Global System forMobile communications (GSM), Enhanced Data rates for GSM Evolution(EDGE), GSM EDGE (GERAN), and the like.

The base station 114 b in FIG. 1A may be a wireless router, Home Node B,Home eNode B, or access point, for example, and may utilize any suitableRAT for facilitating wireless connectivity in a localized area, such asa place of business, a home, a vehicle, a campus, and the like. In oneembodiment, the base station 114 b and the WTRUs 102 c, 102 d mayimplement a radio technology such as IEEE 802.11 to establish a wirelesslocal area network (WLAN). In another embodiment, the base station 114 band the WTRUs 102 c, 102 d may implement a radio technology such as IEEE802.15 to establish a wireless personal area network (WPAN). In yetanother embodiment, the base station 114 b and the WTRUs 102 c, 102 dmay utilize a cellular-based RAT (e.g., WCDMA, CDMA2000, GSM, LTE,LTE-A, etc.) to establish a picocell or femtocell. As shown in FIG. 1A,the base station 114 b may have a direct connection to the Internet 110.Thus, the base station 114 b may not be required to access the Internet110 via the core network 106.

The RAN 104 may be in communication with the core network 106, which maybe any type of network configured to provide voice, data, applications,and/or voice over internet protocol (VoIP) services to one or more ofthe WTRUs 102 a, 102 b, 102 c, 102 d. For example, the core network 106may provide call control, billing services, mobile location-basedservices, pre-paid calling, Internet connectivity, video distribution,etc., and/or perform high-level security functions, such as userauthentication. Although not shown in FIG. 1A, it will be appreciatedthat the RAN 104 and/or the core network 106 may be in direct orindirect communication with other RANs that employ the same RAT as theRAN 104 or a different RAT. For example, in addition to being connectedto the RAN 104, which may be utilizing an E-UTRA radio technology, thecore network 106 may also be in communication with another RAN (notshown) employing a GSM radio technology.

The core network 106 may also serve as a gateway for the WTRUs 102 a,102 b, 102 c, 102 d to access the PSTN 108, the Internet 110, and/orother networks 112. The PSTN 108 may include circuit-switched telephonenetworks that provide plain old telephone service (POTS). The Internet110 may include a global system of interconnected computer networks anddevices that use common communication protocols, such as thetransmission control protocol (TCP), user datagram protocol (UDP) andthe internet protocol (IP) in the TCP/IP internet protocol suite. Thenetworks 112 may include wired or wireless communications networks ownedand/or operated by other service providers. For example, the networks112 may include another core network connected to one or more RANs,which may employ the same RAT as the RAN 104 or a different RAT.

Some or all of the WTRUs 102 a, 102 b, 102 c, 102 d in thecommunications system 100 may include multi-mode capabilities, i.e., theWTRUs 102 a, 102 b, 102 c, 102 d may include multiple transceivers forcommunicating with different wireless networks over different wirelesslinks. For example, the WTRU 102 c shown in FIG. 1A may be configured tocommunicate with the base station 114 a, which may employ acellular-based radio technology, and with the base station 114 b, whichmay employ an IEEE 802 radio technology.

FIG. 1B is a system diagram of an example WTRU 102. As shown in FIG. 1B,the WTRU 102 may include a processor 118, a transceiver 120, atransmit/receive element 122, a speaker/microphone 124, a keypad 126, adisplay/touchpad 128, non-removable memory 130, removable memory 132, apower source 134, a global positioning system (GPS) chipset 136, andother peripherals 138. It will be appreciated that the WTRU 102 mayinclude any sub-combination of the foregoing elements while remainingconsistent with an embodiment.

The processor 118 may be a general purpose processor, a special purposeprocessor, a conventional processor, a digital signal processor (DSP), aplurality of microprocessors, one or more microprocessors in associationwith a DSP core, a controller, a microcontroller, Application SpecificIntegrated Circuits (ASICs), Field Programmable Gate Array (FPGAs)circuits, any other type of integrated circuit (IC), a state machine,and the like. The processor 118 may perform signal coding, dataprocessing, power control, input/output processing, and/or any otherfunctionality that enables the WTRU 102 to operate in a wirelessenvironment. The processor 118 may be coupled to the transceiver 120,which may be coupled to the transmit/receive element 122. While FIG. 1Bdepicts the processor 118 and the transceiver 120 as separatecomponents, it will be appreciated that the processor 118 and thetransceiver 120 may be integrated together in an electronic package orchip.

The transmit/receive element 122 may be configured to transmit signalsto, or receive signals from, a base station (e.g., the base station 114a) over the air interface 116. For example, in one embodiment, thetransmit/receive element 122 may be an antenna configured to transmitand/or receive RF signals. In another embodiment, the transmit/receiveelement 122 may be an emitter/detector configured to transmit and/orreceive IR, UV, or visible light signals, for example. In yet anotherembodiment, the transmit/receive element 122 may be configured totransmit and receive both RF and light signals. It will be appreciatedthat the transmit/receive element 122 may be configured to transmitand/or receive any combination of wireless signals.

In addition, although the transmit/receive element 122 is depicted inFIG. 1B as a single element, the WTRU 102 may include any number oftransmit/receive elements 122. More specifically, the WTRU 102 mayemploy MIMO technology. Thus, in one embodiment, the WTRU 102 mayinclude two or more transmit/receive elements 122 (e.g., multipleantennas) for transmitting and receiving wireless signals over the airinterface 116.

The transceiver 120 may be configured to modulate the signals that areto be transmitted by the transmit/receive element 122 and to demodulatethe signals that are received by the transmit/receive element 122. Asnoted above, the WTRU 102 may have multi-mode capabilities. Thus, thetransceiver 120 may include multiple transceivers for enabling the WTRU102 to communicate via multiple RATs, such as UTRA and IEEE 802.11, forexample.

The processor 118 of the WTRU 102 may be coupled to, and may receiveuser input data from, the speaker/microphone 124, the keypad 126, and/orthe display/touchpad 128 (e.g., a liquid crystal display (LCD) displayunit or organic light-emitting diode (OLED) display unit). The processor118 may also output user data to the speaker/microphone 124, the keypad126, and/or the display/touchpad 128. In addition, the processor 118 mayaccess information from, and store data in, any type of suitable memory,such as the non-removable memory 130 and/or the removable memory 132.The non-removable memory 130 may include random-access memory (RAM),read-only memory (ROM), a hard disk, or any other type of memory storagedevice. The removable memory 132 may include a subscriber identitymodule (SIM) card, a memory stick, a secure digital (SD) memory card,and the like. In other embodiments, the processor 118 may accessinformation from, and store data in, memory that is not physicallylocated on the WTRU 102, such as on a server or a home computer (notshown).

The processor 118 may receive power from the power source 134, and maybe configured to distribute and/or control the power to the othercomponents in the WTRU 102. The power source 134 may be any suitabledevice for powering the WTRU 102. For example, the power source 134 mayinclude one or more dry cell batteries (e.g., nickel-cadmium (NiCd),nickel-zinc (NiZn), nickel metal hydride (NiMH), lithium-ion (Li-ion),etc.), solar cells, fuel cells, and the like.

The processor 118 may also be coupled to the GPS chipset 136, which maybe configured to provide location information (e.g., longitude andlatitude) regarding the current location of the WTRU 102. In additionto, or in lieu of, the information from the GPS chipset 136, the WTRU102 may receive location information over the air interface 116 from abase station (e.g., base stations 114 a, 114 b) and/or determine itslocation based on the timing of the signals being received from two ormore nearby base stations. It will be appreciated that the WTRU 102 mayacquire location information by way of any suitablelocation-determination method while remaining consistent with anembodiment.

The processor 118 may further be coupled to other peripherals 138, whichmay include one or more software and/or hardware modules that provideadditional features, functionality and/or wired or wirelessconnectivity. For example, the peripherals 138 may include anaccelerometer, an e-compass, a satellite transceiver, a digital camera(for photographs or video), a universal serial bus (USB) port, avibration device, a television transceiver, a hands free headset, aBluetooth® module, a frequency modulated (FM) radio unit, a digitalmusic player, a media player, a video game player module, an Internetbrowser, and the like.

FIG. 1C is a system diagram of the RAN 104 and the core network 106according to an embodiment. As noted above, the RAN 104 may employ anE-UTRA radio technology to communicate with the WTRUs 102 a, 102 b, 102c over the air interface 116. The RAN 104 may also be in communicationwith the core network 106.

The RAN 104 may include eNode-Bs 140 a, 140 b, 140 c, though it will beappreciated that the RAN 104 may include any number of eNode-Bs whileremaining consistent with an embodiment. The eNode-Bs 140 a, 140 b, 140c may each include one or more transceivers for communicating with theWTRUs 102 a, 102 b, 102 c over the air interface 116. In one embodiment,the eNode-Bs 140 a, 140 b, 140 c may implement MIMO technology. Thus,the eNode-B 140 a, for example, may use multiple antennas to transmitwireless signals to, and receive wireless signals from, the WTRU 102 a.

Each of the eNode-Bs 140 a, 140 b, 140 c may be associated with aparticular cell (not shown) and may be configured to handle radioresource management decisions, handover decisions, scheduling of usersin the uplink and/or downlink, and the like. As shown in FIG. 1C, theeNode-Bs 140 a, 140 b, 140 c may communicate with one another over an X2interface.

The core network 106 shown in FIG. 1C may include a mobility managemententity gateway (MME) 142, a serving gateway 144, and a packet datanetwork (PDN) gateway 146. While each of the foregoing elements aredepicted as part of the core network 106, it will be appreciated thatany one of these elements may be owned and/or operated by an entityother than the core network operator.

The MME 142 may be connected to each of the eNode-Bs 140 a, 140 b, 140 cin the RAN 104 via an Si interface and may serve as a control node. Forexample, the MME 142 may be responsible for authenticating users of theWTRUs 102 a, 102 b, 102 c, bearer activation/deactivation, selecting aparticular serving gateway during an initial attach of the WTRUs 102 a,102 b, 102 c, and the like. The MME 142 may also provide a control planefunction for switching between the RAN 104 and other RANs (not shown)that employ other radio technologies, such as GSM or WCDMA.

The serving gateway 144 may be connected to each of the eNode Bs 140 a,140 b, 140 c in the RAN 104 via the Si interface. The serving gateway144 may generally route and forward user data packets to/from the WTRUs102 a, 102 b, 102 c. The serving gateway 144 may also perform otherfunctions, such as anchoring user planes during inter-eNode B handovers,triggering paging when downlink data is available for the WTRUs 102 a,102 b, 102 c, managing and storing contexts of the WTRUs 102 a, 102 b,102 c, and the like.

The serving gateway 144 may also be connected to the PDN gateway 146,which may provide the WTRUs 102 a, 102 b, 102 c with access topacket-switched networks, such as the Internet 110, to facilitatecommunications between the WTRUs 102 a, 102 b, 102 c and IP-enableddevices.

The core network 106 may facilitate communications with other networks.For example, the core network 106 may provide the WTRUs 102 a, 102 b,102 c with access to circuit-switched networks, such as the PSTN 108, tofacilitate communications between the WTRUs 102 a, 102 b, 102 c andtraditional land-line communications devices. For example, the corenetwork 106 may include, or may communicate with, an IP gateway (e.g.,an IP multimedia subsystem (IMS) server) that serves as an interfacebetween the core network 106 and the PSTN 108. In addition, the corenetwork 106 may provide the WTRUs 102 a, 102 b, 102 c with access to thenetworks 112, which may include other wired or wireless networks thatare owned and/or operated by other service providers.

Other network 112 may further be connected to an IEEE 802.11 basedwireless local area network (WLAN) 160. The WLAN 160 may include anaccess router 165. The access router may contain gateway functionality.The access router 165 may be in communication with a plurality of accesspoints (APs) 170 a, 170 b. The communication between access router 165and APs 170 a, 170 b may be via wired Ethernet (IEEE 802.3 standards),or any type of wireless communication protocol. AP 170 a is in wirelesscommunication over an air interface with WTRU 102 d.

Enhanced distributed channel access (EDCA) is an extension of the basicdistributed coordination function (DCF) introduced in Institute ofElectrical and Electronics Engineers (IEEE) 802.11 to supportprioritized quality of service (QoS). EDCA supports contention basedaccess of the medium. Carrier sense multiple access with collisionavoidance (CSMA/CA) is an IEEE 802.11 random access protocol in which auser (e.g., wireless transmit/receive unit (WTRU) or station (STA))attempting random access measures the channel to determine whether it isclear prior to transmitting a packet. This random access protocolenables STAs to reduce or eliminate collisions on the channel bypreventing them before they occur.

The point coordination function (PCF) uses contention free channelaccess to support time-bounded services with the AP polling each STA inthe basic service set (BSS). Using the PCF, the AP may send a pollingmessage after waiting a PCF interframe space (PIFS). If the client hasnothing to transmit, the client may return a null data frame. Hybridcoordination function (HCF) control channel access (HCCA) is anenhancement to PCF in which the AP may poll a STA during both acontention period (CP) and a contention-free period (CFP). Using HCCA,an AP may transmit multiple frames under one poll.

Mechanisms for contention-based channel access defined in current IEEE802.11 specifications, such as EDCA and CSMA/CA, only allow one STA toaccess the media at one time. The rest of the STAs in a basic serviceset (BSS) may need to defer channel access and wait for the channelmedium to be cleared. In other words, multi-user concurrent randomaccess is not supported in current IEEE 802.11 specifications. Existingsingle user random access schemes are inefficient and may introducesignificant system delay as compared to multi-user (MU) concurrentrandom access. Embodiments are described herein that provide mechanismsfor MU concurrent random access.

In addition to the limitations of current IEEE 802.11 specificationswith regard to single user concurrent channel access, current IEEE802.11 specifications do not provide for high quality of service forusers, for example, in high density scenarios. However, enhancements arebeing considered for high efficiency wireless local area network (HEW)usage scenarios for a broad spectrum of wireless users, including, forexample, high-density usage scenarios, such as in the 2.4 GHz and 5 GHzband, as well as radio resource management (RRM) technologies. Potentialapplications for HEW include emerging usage scenarios, such as datadelivery for stadium events, high user density scenarios, such as attrain stations or enterprise/retail environments, and usage scenarioswhich evidence shows are becoming increasingly more depended upon, suchvideo delivery and wireless services for medical applications. Inscenarios where there is a dense network with many STAs, the randomaccess procedure may break down due to all the STAs accessing thenetwork simultaneously.

Similarly, evidence has been provided that measured traffic for avariety of applications has a large likelihood for short packets, andthere are also network applications that may generate short packets.Such applications may include, for example, virtual office, transmitpower control (TPC) acknowledgement (ACK) applications, video streamingACK applications, device/controller applications (e.g., mice, keyboards,and game controls), network selection applications (e.g., probe requestsand access network query protocol (ANQP)), and network managementapplications (e.g., control frames).

In the uplink (UL) transmission of a flood of small sized or timesensitive packets, the overhead required to identify STAs with such dataand to schedule them in a typical OFDM or OFDMA transmission may resultin performance degradation due to the overhead of the transmissions.Embodiments described herein may enable efficient transmission of thistype of traffic using OFDMA random access channel (RACH) access. Inscenarios where there are many STAs, such embodiments may also limit oreliminate OFDMA RACH collisions between the different STAs'transmissions.

More specifically, in embodiments described herein, a base station oraccess point (AP) may signal a trigger frame for multi-user channelaccess. As is described in detail below, the trigger frame may trigger aSTA to transmit a UL MU physical layer convergence protocol (PLCP)protocol data unit (PPDU) (e.g., MU-multiple-input multiple-output(MIMO) or OFDMA). The UL MU PPDU may be transmitted in a set of definedresource units (RUs) on which multiple WTRUs or STAs may transmit framesof different types. In embodiments, at least some of the RUs may bedesignated for random access. In some embodiments, a single triggerframe may be signaled per TxOP, or a series of cascading trigger framesmay be signaled per TxOP, for example, to address the scenario wheremany time sensitive or small sized packets are being transmitted in anetwork or BSS at the same time.

By way of example, for an OFDMA UL MU PDDU operating on a 20 MHz band,the building blocks for an OFDMA UL MU response transmission may bedefined as 26-tone with 2 pilots, 52-tone with 4 pilot, and 106-tonewith 4 pilots and with 7 DC Nulls and (6,5) guard tones, and atlocations as illustrated in FIG. 2A. An OFDMA PPDU may carry a mix ofdifferent tone unit sizes within each 242 tone unit boundary. Similarbuilding blocks are defined for 40 MHz, 80 MHz, 160 MHz and 80+80 MHz.

A trigger frame may be used to synchronize and schedule UL MUtransmissions and may serve different purposes, as mentioned above. Tothis end, there may be different types of trigger frames that mayaddress different functions of the system. Further, a trigger frame maybe used to trigger UL MU random access and/or a dedicated transmission.In so doing, it may also facilitate synchronization or time/frequencyalignment of UL transmissions.

For scheduled access, a trigger frame may specify the STAs and theirresource unit (RU) assignment and the transmission parameters per STA inthe HE-SIG-B field in the preamble. The HE-SIG-B field may have a commonfield followed by a user-specific field. The common field may includethe information for all of the designated STAs to receive the PPDU incorresponding bandwidth. The user specific field may include multiplesub-fields that do not belong to the common field, and one or multipleones of the sub-fields may be for each designated receiving STA. Anexample of a user-specific field may include the Station ID (STAID). Forsingle-user allocations in an RU, examples of user-specific fields mayinclude a number of spatial streams (NSTS) field, a transmit beamforming(TxBF) field, a modulation and coding scheme (MCS) field, a dualsub-carrier modulation (DCM) and coding field (e.g., specifying use oflow density parity check (LDCP)). For each user in a multi-userallocation in an RU, examples of user-specific fields may includespatial configuration fields, MCS fields, DCM fields and coding fields.An example HE-SIG-B field 2200 is provided in FIG. 2B.

In embodiments, a trigger frame may be a unicast frame or abroadcast/multicast frame. A unicast trigger frame may have a singlededicated receiver address. Based on the information carried in eitherthe HE-SIG-A and/or HE-SIG-B fields, unintended STAs may not need tomonitor the remaining part of the unicast trigger frame. Abroadcast/multicast trigger frame may not have a single dedicatedreceiver address. Instead, it may have a dedicated or random group ofreceive STAs. The frame may carry scheduling and/or resource allocationinformation. All STAs in range of the transmission may need to monitorthe transmission. Examples of broadcast/multicast trigger frames mayinclude trigger frames for random access and trigger frames that mayschedule UL MU transmission for one or more STAs.

In embodiments, a trigger frame may be aggregated with other dataframes, control frames, or management frames in a medium access control(MAC) layer or in the format of an aggregated MAC PDU (A-MPDU). In thisway, the trigger frame may use the same MCS as other frames. In order tobetter protect the trigger frame, the trigger frame may be allocated asthe first MPDU among the first several MPDUs in an A-MPDU format or thetrigger frame may be repeated in an A-MPDU. The repeated trigger frameMPDU may or may not be allocated adjacently in an A-MPDU. Inembodiments, the original version and repeated version of the triggerframe may be exactly the same, and the version index may be signaled inthe MPDU delimiter, or the version index may be signaled in each MPDU(e.g., using the frame control field).

A trigger frame may be transmitted with other frames any type in a DL MUmode, such as a DL OFDMA mode or a DL MU-MIMO mode. Alternatively, thetrigger frame may be transmitted using a conventional single user (SU)OFDM mode.

In embodiments, a trigger frame may be the acknowledgement of theprevious UL MU frame, or the trigger frame may be aggregated with anacknowledgement frame. In embodiments, the trigger frame or aggregatedtrigger frame may carry physical layer acknowledgements. In embodiments,a trigger frame may be allowed to trigger STAs without associationidentifiers (AIDs).

FIGS. 3A and 3B are diagrams of example trigger frames 300A, 300B,respectively, for MU random access. In both illustrated examples, thetrigger frame may have a unified format, which includes a frame controlfield (FC) 302 a, 302 b, a duration field 304 a, 304 b, an address 1(A1) field 306 a, 306 b, an address 2 (A2) field 308 a, 308 b, a commoninformation field 310, 326, one or more user specific information fields312, 314, 316, 328 and an FCS field 326 a, 326 b. As described in moredetail below, the trigger frame 300A may include acknowledgement/blockacknowledgement (ACK/BA) information in the common information field310, whereas the trigger frame 300B may include an ACK/BA informationfield 338 that is separate from the common information field 326. Theframe structure for the trigger frame 300B may provide added flexibilityin that it may enable the ACK/BA information field 338 to be included oromitted from the frame depending on whether it is needed for thatparticular trigger frame.

For both example trigger frames, the FC field 302 a, 302 b may be usedto indicate that the frame 300A, 300B is a trigger frame. The durationfield 304 a, 304 b may be set to an estimated time duration during whicha UL transmission for an allocated STA is allowed to transmit on the RUsspecified in the trigger frame. The estimated time duration may be incertain units, such as microseconds (ms). Unintended STAs receiving thetrigger frame may set a NAV value for signal protection or multipleprotections. The A1 field 306 a, 306 b may be set to the broadcastaddress or group address if the trigger frame is a broadcast ormulticast frame or to a dedicated receiver MAC address if the triggerframe is a unicast frame. The A2 field 308 a, 308 b may be set to thebasic service set ID (BSSID) associated with the AP, such as the MACaddress of the AP.

For both of the trigger frames 300A and 300B, the common informationfield 310, 326 may include different types of information, such as asequence number and/or trigger token, common transmit power control(TPC) indices, common synchronization information, upcoming SIGinformation, a value of a time synchronization function (TSF) associatedwith the trigger frame and/or beacon sequence, last trigger for PS-POLLinformation, and/or information related to a UL preamble of scheduled ULframes.

The sequence number and/or trigger token may be used to solicit thetrigger frame and/or the upcoming UL MU transmission. Together with theRU index, this information may be used to identify a STA without usingAID or other types of STA IDs. Alternatively, this information may beincluded in the user specific information field. In some embodiments,this information may be omitted depending on the trigger type and/orrandom access type being employed.

The TPC indices may indicate TPC information that may be used by theSTAs for open-loop and/or closed loop TPC. For example, the indices mayinclude the transmit power index that was used to transmit the currenttrigger frame and/or a desired/expected received power index at the APby which multiple STAs may align the received power.

The common synchronization information may include timing and/orfrequency offset correction information. The upcoming SIG informationmay include information to set the L-SIG and/or HE-SIG-A fields in theupcoming UL MU transmission.

Regarding the value of the TSF associated with the trigger frame and/orbeacon sequence, the trigger frame may be used to schedule target waketime (TWT)-enabled STAs, which may not monitor a beacon to adjust theirtiming synchronization functions (TSFs). The TSF information may enablea STA to correct its clock drift to sync up with a future TWT. A beaconsequence may indicate that system information has changed and that STAsmay need to re-read the beacon. STAs may use this information regardlessof whether the STA is addressed in a later user specific informationfield 312, 314, 316, 328.

Regarding the last trigger for PS-Poll information, the trigger framemay be used to schedule TWT-enabled STAs, which may not monitor beacontraffic indication map (TIM) information to know whether they havedownlink (DL) data buffered. A STA that has no UL data and receives atrigger frame indicating that it is the last trigger frame thatschedules UL PS-Polls of the TWT service period (SP) may go to sleep forthe rest of TWT SP. Alternatively, the last trigger for PS-POLLinformation may be included in user specific information field with thecorresponding trigger type and random access type.

Regarding the information related to the UL preamble of the scheduled ULframes, all scheduled STAs may need to construct an HE-SIG-A identicalto each other as there may be no OFDMA for the legacy OFDM symbols. Thisinformation may include information needed for the protection of the DLframes immediately following the UL scheduled frames, such as BA for thescheduled UL frames, or the next trigger frame in a cascading sequence.For example, the AP may dictate how RID in the UL preamble should be setbased on the length of the planned DL frames immediately following thescheduled UL transmission.

The information related to the UL preamble of the scheduled UL framesmay also include a traffic requirement, which may provide informationabout a restriction on the AP added to the random access. The trafficrequirement may be one or more traffic IDs (TIDs), one or more EDCAaccess categories, or one or more traffic categories (TCs). Thisinformation may be included in a field that may, in an embodiment, beimplemented as a hash or bitmap or combination to indicate, for example,one or more TIDs or ACs.

As mentioned above, the trigger frame 300A includes ACK/BA informationin the common information field 310, which may indicate whether thetrigger frame includes acknowledgements for the previously transmittedUL frames and may include MAC ACK/BA information and/or PHY ACK/BAinformation. MAC ACK/BA information may indicate that the one or moreacknowledgements carried is the MAC ACK/BA, which may include an AIDfield that may be set to the AID of the STA of the corresponding datatransmission that the ACK/BA acknowledges. The MAC ACK/BA informationmay also include ACK/BA information, which may be set as a normal ACK orBA field for a previous transmission from the STA with the AIDindicated. PHY ACK/BA information may indicate that the one or moreacknowledgements carried is the PHY ACK/BA, which may not include STAIDs, such as AID or MAC ID. Instead, it may indicate whether atransmission on a certain RU or RUs is successful. The PHY ACK/BAinformation may include an RU index, which may be used to identify theRU, and ACK information, which may indicate whether the informationcarried on the RU is decoded successfully. In embodiments, the PHYACK/BA field may be a bitmap, and each bit may be an ACK/NACKcorresponding to an RU.

For the trigger frame 300B, the common information field 326 includesACK/BA information, which indicates whether the ACK/BA information field338 is present in the trigger frame 300B. The frame 300B furtherincludes the ACK/BA information field 338 later in the trigger frame.The ACK/BA information field 326 is shown in detail in FIG. 3B. If theACK/BA information field 326 is set, it may present either with MACACK/BA or PHY ACK/BA sub-fields, as illustrated in FIG. 3B. If MACACK/BA is present, the ACK/BA information field 338 may include an AIDsub-field 340 and a BA/ACK sub-field 342. The AID sub-field 340 may beset to the AID of the STA with the ACK/BA information. The ACK/BAsub-field 342 may be set as a normal ACK or BA field for a previoustransmission from the STA whose AID is indicated in the AID sub-field340. If PHY ACK/BA is present, the acknowledgement or acknowledgementscarried may be the PHY ACK/BA, which may not include STA IDs, such asAID, MAC ID, etc. Instead, it may indicate whether a transmission oncertain RUs is successful. For PHY ACK/BA, the ACK/BA information field338 may include an RU index sub-field 344 and a BA/ACK sub-field 346.The RU index sub-field 344 may be used to identify the RU on which thetransmission being acknowledged was sent. The BA/ACK sub-field 346 maybe set to indicate whether the information carried on the RU wassuccessfully decoded. Alternatively, the PHY ACK/BA field may be abitmap, and each bit in the bitmap may be an ACK/NACK corresponding to aparticular RU.

Each of the user information fields 312, 314, 316, 238 may includeinformation specific to each respective STA being triggered. More orless user information fields may be included in a trigger frame thanshown in FIGS. 3A and 3B depending on how many STAs are being triggered.Each of the user information fields 312, 314, 316, 328 may includeseveral sub-fields, which may include a STA ID or AID sub-field 318 a,318 b, an RU allocation sub-field 320 a, 320 b, a trigger type sub-field322 a, 322 b, and a trigger information sub-field 324 a, 324 b.

The STA ID or AID sub-field 318 a, 318 b may be set in a number ofdifferent ways. On a condition that a single user or STA is triggered,this field may be set to the AID or other type of STA ID of therecipient. On a condition that a single user is being triggered withoutan AID, such as a STA that has not yet associated with the AP or a STAthat has requested a UL TxOP using a short frame that does not carry aSTA ID, this sub-field may be set to a function of the RU index and asequence number and/or trigger token. Here, the sequence number and/ortrigger token may be used to identify a particular UL MU transmission inthe past, and the RU index may be used to identify the RU used in thatUL MU transmission. In this way, the STA that has transmitted in the RUof the UL MU transmission may be identified. For a group of users/STAs,such as where MU-MIMO is used on the assigned RU, the group may betriggered on a particular RU or multiple RUs, and this sub-field may beset to a group ID, multicast AIDs or other type of IDs that may indicatethe group. On a condition that random access without restriction isbeing triggered, this sub-field may be set to a broadcast ID. On acondition that a random access with restriction is being triggered, thissub-field may be set to a group address, multicast AIDs or any othertype of ID that may indicate a group.

The RU allocation sub-field 320 a, 320 b may be used to assign one ormore RUs to the user/STA.

The trigger type sub-field 322 a, 322 b may identify the type of triggerfor the particular user identified. For example, the trigger may bededicated, which may indicate that a dedicated transmission is beingtriggered for the user. Here, the triggered transmission may be a data,control, or management frame transmission. As another example, thetrigger may be random, which may indicate that a random accesstransmission is being triggered. For another example, the trigger may beinherited, which may indicate that the trigger type is inherited fromanother and/or previous frame type (e.g., management frame). As anotherexample, the trigger type may be mixed, which may indicate that thetrigger frame triggers transmissions that include dedicatedtransmissions and random access transmissions. For example, a triggerframe may explicitly trigger one or more STAs (for example, by includedIDs of STAs and the allocated resources) to transmit on one or moreradio bearers (RBs) or channels. In addition, the trigger frame maytrigger one or more STAs to transmit using random access on one or moreother RBs or channels.

For another example of a trigger type, the trigger type sub-field mayindicate a null data packet (NDP) frame (preamble-only), which mayindicate to a STA, or a group of STAs, that it may send an NDP frame,which may not contain any MAC body. In embodiments, this trigger typemay be used to protect future trigger frames in a cascading sequence. Inembodiments, the AP may decide how early in a cascading sequence tosignal future trigger-frame-protection, for example, because there is atrade-off between spatial reuse and protection. Similarly, this triggertype may trigger the transmission of a common clear to send (CTS) forprotection against legacy overlapping base station subsystem (OBSS)STAs.

FIG. 4A is a system diagram 400A illustrating a potential scenario thatmay cause a collision that may be remedied by an NDP-type trigger. Inthe example illustrated in FIG. 4A, STAs 405 a and 410 a may transmit ULframes 440 a and 442 a, respectively, to an AP 415 a based on scheduledRUs and a length of transmission indicated in a first trigger frame 435a in a sequence of cascading trigger frames, or in a random access wayusing allocated RUs in the first trigger frame 435 a. An OBSS STA 425 acannot hear the AP 415 a or STAs 405 a and 410 a, and it transmits aframe 450 a to an OBSS AP 430 a. The AP 415 a may not be aware of thetransmission 450 a of the OBSS STA 425 a and may send a trigger frame445 a to another STA 420 a. As illustrated in FIG. 5, in this scenario,the trigger frame 445 a may collide at the STA 420 a with thetransmission 450 a of the OBSS STA 425 a.

FIG. 4B is a system diagram 400B illustrating how use of the NDP-typetrigger may prevent the collision in the potential scenario of FIG. 4A.In the example illustrated in FIG. 4B, the AP 415 b may schedule an NDPframe transmission 417 from the STA 420 b in the first trigger frame 435b that also schedules the data frame transmissions 440 b and 442 b fromthe STAs 405 b and 410 b. The NDP frame 417 may end at HE-SIG-A and mayinclude a response indication (RID) indicating the length of theexpected DL response or responses following the UL transmissions 440 band 442 b of the STAs 405 b and 410 b. On a condition that the OBSS STA425 b hears the NDP frame 417 from the STA 420 b, the OBSS STA 425 b mayuse the RID information to defer channel access such that it will notinterfere with reception of the DL trigger frame 445 b at the STA 425 b.In the following UL data frame transmission from the STA 420 b, the RIDin the preamble may include any necessary information to extend theprotection for the STA 420 b to receive acknowledgement for itstransmission.

Referring back to FIGS. 3A and 3B, the trigger information sub-field 324a, 324 b of the user information field 312, 314, 316, 328 may includedetailed triggering information and may have variable sizes, forexample, depending on the trigger frame type.

On a condition that the trigger type is dedicated trigger, the triggerinformation sub-field 324 a, 324 b may include a dedicated access type,an MCS, a number of spatial streams (Nss) or spatial time streams(Nsts), transmit power control information, timing correctioninformation, frequency correction information, maximum packet size inunits (e.g., upcoming UL PPDU length in OFDM symbols or upcoming UL MPDUor A-MPDU size in bytes), coding scheme (BCC or LDPC), ACK policy, guardinterval size, HE-LTF type, number of HE-LTFs in the upcoming ULtransmission, and/or HE-SIG-A type. With regard to the dedicated accesstype, for example, any of the following dedicated access types may bedefined: dedicated access for acknowledgement (D-ACK) and dedicatedaccess for traffic poll (D-TP). The D-TP type of dedicated trigger framemay be used by the AP to poll STAs for traffic information and status.The trigger type may also be STA-specific, and the IDs of the specificSTAs triggered to transmit may be explicitly or implicitly included inthe trigger frame.

On a condition that the trigger type is random trigger, the triggerinformation sub-field 324 a, 324 b may include a random access typeand/or a random trigger body. With regard to the random access type, forexample, any of the following random access types may be defined: randomaccess for initial link setup (R-Initial), random access for powersaving STAs (R-PS), random access for traffic poll (R-TP), or randomaccess for time sensitive small data transmission (R-SD). The R-initialrandom access type of trigger frame may be used to trigger STAs that maytry to associate with the AP. The R-PS random access type of triggerframe may be used for STAs that may wake up from a sleep mode. The R-TPrandom access type of trigger frame may be used by the AP to poll STAsfor traffic information and status. The R-SD random access type oftrigger frame may be used by the AP to allocate a time slot for fast ULsmall data transmissions. The trigger body may be a sub-field that mayvary in size depending on the defined random access type for the triggerframe.

The trigger frame illustrated in FIG. 3A may allow an AP to triggerdifferent types of transmissions. For example, an AP may have a 20 MHzchannel with 9 RUs. The AP may, for example, allocate RUs 1-3 forinitial random access by setting the user information field 312 as:AID=0 (indicating a group address); RU allocation: RUs 1-3; TriggerType=‘Random’. In the trigger information sub-field 324, the randomaccess type may be set to R-Initial. In this example, the AP mayallocate RUs 4-8 for random access traffic polling by setting the userinformation field 314 as: AID=0 (indicating a group address); RUallocation: RUs 4-8; Trigger Type=‘Random.’ In the trigger informationsub-field 324 a, the random access type may be set to R-TP. The AP mayallocate RU 9 to STA k for UL data transmission by setting the userinformation field 316 as: AID=STA k's MD; RU allocation: 9; TriggerType=‘Dedicated.’

In embodiments, the frame format illustrated in FIGS. 3A and/or 3B maybe used to define a new control frame. However, one or more of thefields, such as the common information field 310, 326 the user specificinformation field 310, 312, 316, 328, and/or the ACK/BA informationfield 338, may be aggregated in any frame that may carry the MU controlinformation.

A random access frame may be transmitted in response to the triggerframe that allocated at least one RU for random access. A random accessframe may be a MAC frame and may have different formats depending on thetrigger type and random access type indicated in the trigger frame. On acondition that the trigger type is set in the trigger frame as ‘Random’and the random access type is set to Type=‘R-Initial’; indicating thatrandom access is for initial setup, the random access frame may be aprobe request frame, an association request frame, a reassociationrequest frame or other type of initial link setup related frame. On acondition that the trigger type is set in the trigger frame as ‘Random’and the random access type is set to Type=‘R-PS,’ indicating that randomaccess is for power saving STAs, the random access frame may be aPS-Poll frame or other type of power saving related frame. On acondition that the trigger type is set in the trigger frame as ‘Random’and the random access type is set to Type=‘R-TP,’ indicating that randomaccess is for traffic poll, the random access frame may be a UL responseframe or other type of frame to indicate the UL traffic status. On acondition that the trigger type is set in the trigger frame as ‘Random’and the random access type is set to Type=‘R-SD,’ indicating that randomaccess is for time sensitive small data transmission, the random accessframe may be a UL data packet. Certain restrictions may be applied tothe data packet transmission. For example, the packet size and/ortraffic type may be restricted.

In embodiments, the random access frame may be a short random access(SRA) frame, which may be defined particularly for UL MU random access.The SRA frame may be a MAC frame or a PHY frame. As described above withrespect to FIGS. 4A and 4B, there may be scenarios under which therandom access protocol for UL MU random access may not completely avoidcollisions. The SRA frame may be used to protect long packettransmissions from collision.

FIG. 5 is a diagram of an example SRA MAC frame 500. The SRA MAC frame500 may be a MAC control frame or a MAC management frame, which may betransmitted in any SU PPDU or MU PPDU. In this way, the AP, as areceiver of the SRA frame 500, may acquire the MAC address of the STAs.Thus, for a cascading trigger frame (which will described in more detailbelow), the AP may allocate the UL resources using MAC address orcorresponding STA ID (e.g., AID or PAID).

The example SRA MAC frame 500 illustrated in FIG. 5 includes an FC field505, a duration field 510, an RA field 515, a TA field 520 and an FCSfield 525. The frame control field 505 may indicate that the frame is anSRA frame using type or subtype sub-fields in the FC field 505. Theduration field 510 may be used to set a NAV for unintended STAs that mayprotect up to the end of a sequence of multiple frames (e.g.,multi-station BA as will be described in more detail below). The RAfield 515 may include the address of the STA. The TA field 520 mayinclude the MAC address of the AP and may be omitted in somecircumstances.

Alternatively, an existing control frame or management frame may bereused or re-interpreted as an SRA frame. For example, an RTS frametransmitted by a non-AP STA in an RU that may be allocated for UL randomaccess may be considered as an RA frame. The AP, as a receiver of theRTS frames, may treat them as SRA frames instead of normal RTS frames.

In embodiments, a trigger frame may allocate all of the RUs for UL MUrandom access. In this case, an NDP or semi-NDP SRA frame may be used.The NDP or semi-NDP SRA transmission may be considered as a UL MU PPDUwithout a MAC body. The SIG field in the PLCP header may be over-writtenas an SRA frame. An NDP SRA frame may take one of a number of differentforms, examples of which are provided in FIGS. 6A, 6B and 6C.

FIG. 6A is a diagram of an example NPD SRA frame 600A. The example NDPSRA frame 600A illustrated in FIG. 6A includes L-STF and L-LTF fields602 a and 604 a, respectively, which may be prepared as for a normaltransmission. The NDP SRA frame 600A may also include an L-SIG field 606a, which may be prepared according to the instructions provided in thetrigger frame, as described above. A length field in the L-SIG field 606a may indicate the length of the current SRA transmission. With the NDPtransmission, the length may be less than the non-NDP frame such that aSTA may notice that this is an NDP frame. In embodiments, not all of thenon-AP STAs that transmit in the UL MU random access frame may have thesame L-SIG field 606 a.

The example NDP SRA frame 600A may also include HE-SIG-A1 and HE-SIG-A2fields, 608 and 612, respectively. The HE-SIG-A1 field 608 may be thefirst half of the HE-SIG-A field prepared according to the instructionsin the trigger frame, as described above. The HE-SIG-A1 field 608 mayhave the length of an integer number of OFDM symbols. Some fields in theHE-SIG-A1 field 608 may indicate that this frame is an NDP SRA frame.Not all of the non-AP STAs that transmit in the UL MU random access mayhave the same HE-SIG-A1 field. The HE-SIG-A2 field 612 may be the secondhalf of the HE-SIG-A field and may be prepared according to theinstructions in the trigger frame, as described above.

FIG. 6B is a diagram of another example NPD SRA frame 600B. As for theexample NDP SRA frame 600A, the example NDP SRA frame 600B illustratedin FIG. 6B includes L-STF and L-LTF fields 602 b and 604 b,respectively, which may be prepared as for a normal transmission. As forthe example NDP SRA frame 600A, the NDP SRA frame 600B may also includean L-SIG field 606 b, which may be prepared according to theinstructions provided in the trigger frame, as described above. Thelength field in the L-SIG field 606 b may indicate the length of thecurrent SRA transmission. With the NDP transmission, the length may beless than the non-NDP frame such that a STA may notice that this is anNDP frame. In embodiments, not all of the non-AP STAs that transmit inthe UL MU random access frame may have the same L-SIG field 606 b.

The UL MU random access frame may be a direct response to the triggerframe. The SRA transmission may be scheduled by the AP, and the SIGfields may be assigned (or dictated) by the AP. On a condition that theAP expects the NDP SRA frame, the STAs may not need to explicitly signalthe NDP SRA frame in its UL transmission. For the example NPD SRA frame600B, instead of having separate HE-SIG-A1 and HE-SIG-A2 fields carryingfirst and second halves of the HE-SIG-A field, as was the case for theexample NPD SRA frame 600A, the STA may overwrite the HE-SIG-A2 fieldusing a user specific sequence to form the HE-SIG-A field 614 a. TheL-STF/L-LTF and L-SIG fields may, however, be the same among all users.

FIG. 6C is a diagram of another example NDP SRA frame 600C. The NDP SRAframe may be further simplified if backward capability may be addressedby the trigger frame. For example, the trigger frame may be detected bya legacy frame, which may set a NAV for the following multiple frameexchanges. For the example NDP SRA frame 600C illustrated in FIG. 6C, anL-STF field 602 c may be needed for automatic gain control (AGC) andtiming/frequency detection. However, an LTF field may or may not beincluded in the NDP SRA frame 600C. The NDP SRA frame 600C illustratedin FIG. 6C includes an HE-SIG-A field 614 b following the training fieldor fields. As for the example NDP SRA frame 600B, the HE-SIG-A field 614b may be over-written and may include a user specific sequence only.

For the example NDP SRA frames 600B and 600C that use user specificsequences, the sequences may be orthogonal to each other such that theAP may distinguish them even when they are transmitted concurrentlyusing the same frequency-time resources. Each sequence may have asequence ID associated with it. Thus, the AP may use the sequence ID toindicate the STA that successfully transmitted the SRA using thatsequence. The user specific sequence may be assigned by the AP in aBeacon frame, an Association response or other type of frame.Alternatively, the STA may randomly select the sequence from a set ofsequences, which may be, for example, specified in the standards.

Instead of using orthogonal sequences, an SRA frame may have anOFDMA-like format, which may be referred to as a semi-NDP SRA frame.FIG. 7 is a diagram of an example semi-NDP SRA frame 700. The examplesemi-NDP SRA frame 700 illustrated in FIG. 7 includes L-STF, L-LTF,L-SIG and HE-SIG-A fields 702, 704, 706 and 708, respectively, in thePLCP header, which may not be over-written. An SRA field 710 may betransmitted after the preamble on a per RU basis. However, the SRA field710 may not be a full MAC frame. Instead, the SRA field 710 may carrylimited information, such as HE-STF, HE-LTF and SRA information 712,714, and 716, respectively, which may be short in the time domain (e.g.,one OFDMA symbol long).

For example, the minimum RU size may be 26 subcarriers per OFDMA symbol.Here, then, the basic SRA field 710 may carry 26 coded or uncoded bits.In embodiments, the SRA field may be a common sequence or a userspecific sequence. In other embodiments, the SRA field may be designedto carry some information, such as a compressed STA ID.

As described briefly above, acknowledgement for a UL random access framemay be included in a trigger frame, which may be used to acknowledge theprevious UL transmission and trigger a new UL transmission. Inembodiments, such acknowledgement may be made in the ACK/BA informationfield (or as ACK/BA information in the common information field) in thetrigger frame, or the acknowledgement frame may be aggregated with otherframes in an A-MPDU format. In embodiments, a multi-STA BA frame may beused. The acknowledgement may be a MAC layer acknowledgement or a PHYlayer acknowledgement, as described above. For example, PHY ACK/BA maybe more suitable for use than MAC ACK/BA when SRA is used at leastbecause the SRA may not include any information such as MAC address, AIDor other type of STA ID. For another example, PHY ACK/BA may be moresuitable than MAC ACK/BA when a trigger frame is used to trigger initiallink setup when the AID may not be set to the STA.

STAs and APs may indicate their capabilities to support UL MU randomaccess. For example, the AP may include an indicator that the AP iscapable of UL MU random access in its Beacon, Probe, Response,Association Response frames or other type of frame or in the MAC headeror PLCP header. Similarly, STAs may indicate capability to support UL MUrandom access in their Probe Request, Association Request or othermanagement, control or other type of frame or in the MAC header or PLCPheader.

In embodiments, a ‘UL MU random access support’ subfield may be includedin the Capability information field or in a new HE Capabilityinformation element (IE). Alternatively, several separate random accesscapability indicators may be defined for different uses, such asCapability of supporting random access for initial access, Capability ofsupporting random access in power saving mode, Capability of supportrandom access for traffic poll and Capability of supporting randomaccess for time sensitive small packets.

FIGS. 8A and 8B are signal diagrams of example frame exchanges withrandom access and trigger frames. In FIGS. 8A and 8B, four RUs areprovided as an example. However, more or less RUs may be used, as wouldbe recognized by one of ordinary skill in the art.

FIG. 8A is a signal diagram 800A of an example frame exchange between anAP 805 a and STAs 850 a where an AP 805 a may transmit a trigger frame810, and the trigger frame 810 allocates RUs 1-3 for random access andRU 4 as dedicated for STA3 of the STAs 850 a. A short interface space(SIFS) time after the trigger frame 810, STAs 850 a may transmit UL MUframes in a UL MU transmission 812. Two of the STAs 850 a, STA1 andSTA2, may transmit random access frame 815 and 820 on RU1 and RU2,respectively. The AP 805 a may detect these transmissions withoutcollision. In the example illustrated in FIG. 8A, two or more of theSTAs 850 a transmitted their random access frame 825 on RU3, but thetransmissions collided. STA3 may transmit a dedicated frame 830 on theallocated RU, RU4.

In the next DL transmission frame (a SIFS time after the UL MUtransmission 812), the AP 805 a may transmit A-MPDUs 832, whichaggregate a block ACK (BA) response 835 and 840 to random access frame815 and 820 on RU1 and RU2, respectively. A new trigger frame 842 may betransmitted on RU3 where the collision previously occurred. On RU4, theAP 805 a may transmit a BA frame 844 to STA3, which may be aggregatedwith a unicast trigger frame 846 to STA3 to trigger another ULtransmission from STA3.

FIG. 8B is a signal diagram 800B of another example frame exchangebetween an AP 805 b and STAs 850 b. A difference between the examplesillustrated in FIGS. 8A and 8B is that, in the example illustrated inFIG. 8B, in the second DL transmission 855, instead of a unicast BA, amulti-STA BA frame 858 may be used. As shown in the example of FIG. 8B,the AP 805 b may transmit the multi-STA BA 858 as an acknowledgement toSTA1, STA2 and STA3 for the received and successfully decodedtransmissions 815, 820 and 830. The AP 805 b may also transmit a triggerframe 860 on RU2 and RU3 and a data frame 865 to a new STA, e.g. STA4,on RU4.

FIG. 9 is a flow diagram 900 of an example method of triggering UL MUrandom access. In embodiments, an AP may acquire the channel mediumeither through contention or scheduling. In the example illustrated inFIG. 9, once the AP acquires the channel, it transmits a trigger frame(910), which may include allocation of at least one OFDMA building blockor RU for random access in the upcoming UL OFDMA transmission. The APmay transmit the trigger frame in one of many different ways. Forexample, the AP may transmit the trigger frame as a standalone frame(e.g., the trigger frame 810 illustrated in FIGS. 8A and 8B). As anotherexample, the trigger frame may be a MAC frame and may be aggregated withother frames (e.g., including one or more data frames, control framesand management frames) using an A-MPDU format. In this example, thetransmission of the trigger frame may be in an OFDM mode, an OFDMA modeor other mode. An example of this type of trigger frame is the triggerframe 846 illustrated in FIG. 8). For another example, the AP maytransmit the trigger frame and other frames (such as data, controland/or management frames) in an MU mode (DL OFDMA or other MU mode).Examples of this type of trigger frame are the trigger frames 842 and860 in FIGS. 8A and 8B.

If the trigger frame is transmitted in DL OFDMA mode, the resourceallocation field in SIG-B of the DL MU PPDU that carries the triggerframe may use a broadcast/multicast (e.g., group ID, extended group ID,multicast AID) or unicast ID (e.g., PAID) to indicate that certain RUsmay be assigned for trigger frame transmission. When a broadcast ormulticast ID is used, the corresponding one or more potential recipientsand/or STAs may need to detect the trigger frame, and unintended STAsmay skip the detecting of the trigger frame.

Referring back to FIG. 9, after a SIFS time, the AP may receive ULtransmissions from multiple STAs (920). On each RU assigned for randomaccess, for example, the AP may successfully receive a single randomaccess packet from a STA, may receive multiple random access packetsfrom more than one STA (which cause a collision on the RU), or mayreceive nothing on the particular OFDMA building block.

A SIFS time after receiving the UL MU random access transmission, the APmay transmit one or more acknowledgement frames (930) to the STAs. Theacknowledgement frames may include, for example, multi-STA BA frames,ACK frames and/or BA frames. In embodiments, the AP may cascade theacknowledgements with other DL data, control and/or MAC frames. The APmay also aggregate the ACK with a trigger frame, which may be used, forexample, to trigger a new set of UL transmissions.

FIG. 10 is a flow diagram 1000 of an example method of UL MU randomaccess. In the example illustrated in FIG. 10, a STA may detect atrigger frame that assigns at least one OFDMA building block or RU forUL MU random access in the upcoming UL OFDMA transmission (1010). On acondition that the DL transmission from the AP is in an OFDMA mode, theSTA may check the SIG-B field for a resource allocation for the triggerframe. On a condition that the STA has a UL frame to transmit (e.g., adata, management or control frame) and the STA meets any restrictions orrequirements specified in the received trigger frame (1020), the WTRUmay prepare its UL transmission in the assigned UL random access RU(1030). This may include, for example, applying any necessary padding,power adjustment and synchronization adjustment to the UL transmissionsuch that the transmission from multiple STAs will be aligned at the AP.

FIG. 11 is a flow diagram 1100 of another example method of UL MU randomaccess. In the example illustrated in FIG. 11, the STA generates aseparate random backoff index R for a deferral index on random access RUselection before transmitting (1105). In embodiments, the random numberR may be drawn from a uniform distribution over the interval [C, CI],where CI is the contention index, which may be in the range of [CImin,CImax]. CI may initially be set to CImin.

The WTRU may then compare R with M (M is the number of RUs that wereallocated for random access in the detected trigger frame). On acondition that R≤M (1110), the WTRU may transmit on the Rth RU assignedfor random access (1115). On a condition that R>M (1110), the WTRU mayhold its transmission (1120), reset R=R−M (1125), and compete for thenext UL MU random access opportunities using the reset offset value R(1130).

Referring back to FIG. 10, a SIFS time after the WTRU transmits itsrandom access frame, if the transmitted frame is successfully receivedat the AP, the WTRU may receive an ACK frame from the AP (1040).Alternatively, a new trigger frame for future dedicated or random accessUL MU transmission received in the DL may serve as acknowledgement for aUL random access transmission. On a condition that the acknowledgementis received from the AP, indicating that the random access frame issuccessfully decoded at the AP side, then the WTRU may set CI=CImin.Otherwise, the STA may set CI=min(CImin*2,CImax).

In embodiments, such as for dense STA deployment and/or where a largenumber of small sized or time sensitive packets are being transmitted ina BSS at the same time, an AP may transmit a number of trigger framesper TxOP. FIGS. 12, 13 and 14 provide examples where multiple triggerframes are transmitted in a TxOP.

FIG. 12 is a diagram 1200 of a random access procedure with SRA frames.In the example illustrated in FIG. 12, an AP 1202 transmits a triggerframe 1205, which may allocate at least one RU for random access. A SIFStime after the trigger frame 1205, the STAs may transmit UL MU frames ina UL MU transmission 1220. In the UL MIMO transmission 1220, STAs 1 and2 of the STAs 1250 transmit SRA frames 1225 and 1230 on RU1 and RU2,respectively. The SRA frames may be, for example, any of the SRA framesillustrated in FIGS. 6A, 6B and 6C and may include very limited MACinformation, if any. A collision may occur between frames 1235transmitted on RU 1235, and no frame 1240 may be transmitted on RU4,which may, accordingly, be empty.

The AP may send another trigger 1210 during the TxOP, schedulingdedicated UL transmissions for the users that successfully transmittedthe SRA frames (STA1 and STA2 in this example) in the UL MU transmission1220. In the example illustrated in FIG. 12, STA1 is assigned RUs 1 and2 for its dedicated transmission and STA2 is assigned RUs 3 and 4 forits dedicated transmission. The trigger frame 1210 may also include oneor more of PHY/MAC ACK/NACK frames. A SIFS time after the trigger frame1210, another UL MU transmission 1260 may occur, with the STAs 1 and 2transmitting dedicated frames 1245 and 1255, respectively. Inembodiments, the duration frame in the frame exchanges illustrated inFIG. 12 may be set to protect a sequence of multiple frames.

From the STA end, in response to receiving the trigger frame 1205, itmay check whether it may transmit. On a condition that the STAdetermines that it may transmit a UL MU random access frame, the STA mayprepare L-STF, L-LTF fields as in a normal transmission and prepare theL-SIG field following the instruction in the trigger frame 1205. The STAmay prepare the HE-SIG-A1 field, which may be the first half of theHE-SIG-A field as described above with respect to FIGS. 6A, 6B and 6C,following the instruction in the trigger frame 1205. In embodiments, theSTA may overwrite the HE-SIG-A2 field, which may be the second half ofthe HE-SIG-A field, using a user specific sequence.

For scenarios where there may be small packets, such as trafficindication packets informing the AP that specific STAs have packets tosend, or time sensitive packets, such as packets carrying VoIP or gamingcontrol traffic, the AP may set a random access window with N differentrandom access opportunities, which may be referred to as a continuousrandom access transmission opportunity (CRA TxOP).

In embodiments, an initial random access trigger frame in a series ofcascading trigger frames in a random access transmission opportunity mayindicate the traffic type, size and related information as well as thenumber of trigger frames in the CRA TxOP with a combined multi-STA blockACK/trigger frame inter-spaced between each random access opportunity(RaOP) transmission. In other embodiments, each new RaOP may betransmitted a SIFS time after the previous one ends. Here, a delayedMU-block ACK may be transmitted from the AP at the end of the TxOP.Inter-spaced trigger frames may add and/or remove random access RUswithin the CRA TxOP by scheduling specific users on specific RUs on anas-needed basis. Different embodiments for cascading trigger frames aredescribed in more detail below with respect to FIGS. 13 and 14.

To determine the number of random access RUs needed, the AP may observethe number of empty random access RUs within a transmission. This may bea function of no STAs accessing the channel and the number of collisionsthat occur within a resource. A feedback subframe that informs the AP ofthe number of random access collisions that the STA has had as well asthe primary channels these collisions have occurred on may also be usedto help dimension the allocation. In this case, the feedback may includeboth the RAB index and the RU index within the RaOP.

In embodiments, the STAs may be classified into different groups andrandom access may be limited to a specific group. This may be combinedwith cascaded OFDMA transmission to ensure that all the groups are givenan opportunity to access the channel. In other embodiments, the STAs maybe grouped and specific groups may access a specific set of OFDMAresources.

By way of example, in scenarios where there are many collisions, the APmay specify which groups of STAs are permitted to access a specificRaOP. For example, a first STA group (STA group 1) may be allowed toaccess RaOP 1, a second STA group (STA group 2) may be allowed to accessRaOP 2, and so on. In this case, signaling may be needed to group theSTAs and identify the random access opportunity each group may use. ASTA may belong to multiple groups. RaOPs may be coordinated betweenoverlapping BSSs to limit the effect of OBSS collisions between theBSSs.

FIG. 13 is a signal diagram 1300 of an example sequential random accessprocedure using cascading trigger frames where RaOPs occur afterinter-spaced block ACK/trigger frames. In the example illustrated inFIG. 13, an AP 1310 sends a trigger frame 1304, which may be a Trigger-Rframe that may carry one or more of the duration of the CRA TxOP, thetriggered traffic size, the triggered traffic type, the number of RaOP(N) in the CRA TxOP and an indication that the RaOPs occur after aninter-spaced block ACK/trigger frame, such as MU-BA-Trigger frame 1306.

In embodiments, the trigger frames, such as Trigger-R frame 1304 and/orthe MU-BA/trigger frames 1306, may indicate which STAs are permitted totransmit in the RaOP and/or the CRA TxOP. In one example, all STAs maybe permitted to access any random access resources within the CRA TxOP.In another example, the trigger frame may indicate a sequence of STAgroups permitted within each RaOP. Here, the AP may divide the spaceinto multiple random access groups, where each random access groupincludes STAs with some commonality (e.g., traffic size, physicalproximity, and common MCS requested to reduce padding when traffic ofthe same size is transmitted). STAs may be allowed to access the randomaccess channel in the RaOP they are assigned to. Some RaOP may be leftfor random access of the STAs at the same time. For example, RaOP 1=STAgroup 1, RaOP 2=STA group 2, and RaOP n=all STAs.

Further, the trigger frames, such as Trigger-R frame 1304 and/or theMU-BA/trigger frames 1306, may indicate the resources dedicated torandom access. In one example, all RUs in the CRA TxOP may be allocatedfor random access. In another example, only a subset of the RUs in thetransmission bandwidth may be allowed for random access. This subset ofresources may be constant over an entire CRA TxOP or may change over thecourse of the CRA TxOP.

STAs may perform random access in the random access resources/RaOPs thatthey are permitted to access. For example, as illustrated in FIG. 13,STAs may transmit frames during transmissions 1312, 1322, etc. in a CRATxOP 1302. In the illustrated example, after Trigger-R frame 1304, STA1transmits a random access frame 1314 on RU1, STA2 transmits a randomaccess frame 1316 on RU2, STAs 3 and 4 transmit random access frames1318 on RU3, which collide, and RU4 is empty (1320). As mentioned above,a block ACK 1306 may be sent to the STAs immediately after thetransmission, and the inter-spaced trigger frames 1306 may add and/orremove random access RUs within the CRA TxOP 1302 by scheduling specificusers on specific RUs on an as-needed basis.

FIG. 14 is a signal diagram 1400 of an example sequential random accessprocedure using cascading trigger frames where RaOPs occur immediatelyafter each other with a delayed MU-Block ACK. The example illustrated inFIG. 14 is the same as the example illustrated in FIG. 13 except that,instead of the AP 1310 transmitting block ACKs immediately after eachtransmission, RaOPs occur immediately after each other in the CRA TxOP1302 with a delayed MU-Block ACK 1360 being transmitted to acknowledgeall transmissions sent during the CRA TxOP 1302 at the end of thesequential TxOP 1302. The delayed block ACK 1360 may be based on any ofthe block ACK embodiments described above, but additional informationregarding the RaOP the data was transmitted on may need to be includedin the delayed block ACK 1360.

FIG. 15 is a signal diagram 1500 of an example PHY layer acknowledgementprocedure when delayed ACK is permitted. In the example illustrated inFIG. 15, after acquiring the channel medium, the AP 1502 may transmit atrigger frame 1504, which may include a token or sequence number 1506.The token or sequence number may be increased in each subsequent triggerframe, as shown with respect to the token 1510 in the trigger frame1508. On a condition that the token or sequence number reaches a maximumallowed threshold, token_max, the AP 1502 may reset the token/sequenceto the initial value. Alternatively, the token/sequence may be includedin other types of DL frames that may be followed by a UL MU frame.

The AP 1502 may receive random access or SRA frames 1512, 1516 in the ULMU frame a SIFS time after each trigger frame 1504, 1508. The AP 1502may record the RU indices on which random access frames weresuccessfully detected.

In one embodiment illustrated in FIG. 15, a SIFS time after the UL MUframe 1512, the AP may include a PHY layer acknowledgement in the DLtransmission 1512, for example. This acknowledgement may be an immediatePHY ACK/BA for the previous UL MU transmission 1512, and the AP may usethe RU index to explicitly indicate the transmission that it isacknowledging. In another embodiment illustrated in FIG. 15, somecombination of delayed and immediate PHY ACK/BA may be used, or justdelayed PHY ACK/BA may be used, to acknowledge the past several MUtransmissions. Here, the AP 1502 may use the token included in thetrigger frame and the RU index to explicitly indicate the transmission.If PHY ACK/BA is used, the AP 1502 may transmit the frame in a broadcastformat (e.g., the SIG-B field may indicate that the frame may need to bedecoded by all STAs 1520. The maximum allowed delayed time in units ofDL/UL frame exchanges may be less than Σ_(n=1) ^(token) ^(_) ^(max)T_(n), where T_(n) may be a transmission duration of the nth triggerframe plus the following UL transmission. In embodiments, both PHY layerACK and NACK may be signaled.

FIG. 16 is a flow diagram 1600 of an example method of UL MU randomaccess. The method may be implemented in a non-AP STA or other WTRU, forexample, using some combination of a transmitter, receiver and one ormore processors. In the example illustrated in FIG. 16, a trigger framefor UL MU transmission is detected (1610). In embodiments, the triggerframe may include an assignment of RUs for random access in upcoming ULMU PPDUs and an indication that the trigger frame is one of a pluralityof trigger frames in a cascading sequence of trigger frames in an MUtransmission opportunity (TxOP). One of the RUs in the assignment of RUsmay be selected for a random access transmission (1620). A random accesstransmission may be sent on the selected RU (1630). An ACK for therandom access transmission may be received on a condition that thetransmission is successfully received and decoded (1640), for example,by an AP, base station or other WTRU. In embodiments, the ACK may beaggregated with one of the plurality of trigger frames in the sequence.

In embodiments, at least one trigger frame of the plurality of triggerframes in the sequence may include a last trigger indication. Inembodiments, the last trigger indication may indicate, for example, thatthe at least one frame is the last trigger frame in a target wait time(TWT) service period (SP) that schedules UL power save polls (PS-polls).A sleep state may be entered in response to the indication in the atleast one trigger frame. The trigger frame may include a field thatpolls for a traffic buffer status of the WTRU.

The assignment of RUs for random access in the upcoming UL OFDMAtransmission may be for an integer number, M, of RUs, and the one of theRUs in the assignment of RUs for random access transmission may beselected by generating a random backoff index, R, from a range ofinteger values between zero and defined maximum value. On a conditionthat R>M, the random access transmission may be held, and the value of Rmay be reset to R=R−M. The reset value of R may be used to compete forthe next MU random access opportunity in the TxOP. The non-AP STA orWTRU may transmit on an Rth RU of the RUs included in the assignment ofRUs for random access on a condition that R≤M. The trigger frame mayinclude a common information field that carries information to set ahigh efficiency (HE)-SIG-A field in the upcoming UL MU PPDU.

FIG. 17 is a flow diagram 1700 of another example method of UL MU randomaccess. The method may be implemented in an AP, base station or otherWTRU, for example, using some combination of a transmitter, receiver andone or more processors. In the example illustrated in FIG. 17, a triggerframe for UL MU transmission may be generated and transmitted to aplurality of WTRUs or STAs (1710). In embodiments, the trigger frame mayinclude an assignment of RUs for random access in upcoming ULtrigger-based PPDUs and an indication that the trigger frame is one of aplurality of trigger frames in a cascading sequence of trigger frames inan MU TxOP. The UL trigger-based PPDUs may then be received (1720). Inembodiments, the UL trigger-based PPDUs may include at least one framefrom at least one of the plurality of WTRUs or STAs on one of theassigned RUs. An acknowledgement for the at least one frame may betransmitted (1730) on a condition that the at least one frame wassuccessfully received and decoded. In embodiments, the ACK may beaggregated with one of the plurality of trigger frames in the sequence.

In embodiments, at least one of the plurality of trigger frames in thesequence may include a last trigger indication that may trigger WTRUs orSTAs to enter a sleep state. In embodiments, the last trigger indicationmay be, for example, an indication that the trigger frame is the lasttrigger frame in a target wait time (TWT) service period (SP) thatschedules UL power save polls (PS-polls). The trigger frame may includea field that polls for a traffic buffer status of WTRUs or STAs. Inembodiments, the trigger frame may include a common information fieldthat carries information to set an HE-SIG-A field in the UL MU PPDU.

Once a non-AP STA or WTRU has made the AP aware of its need for ULaccess, and after any of the UL MU random access procedures describedabove, or after a scheduled UL transmission of a short packet (e.g., byproviding buffer status information in a PS-Poll frame per AC), a STAmay still access the medium via the normal EDCA procedure or via therandom access trigger frame. FIG. 18 is a diagram 1800 illustrating apotential collision that may occur as a result of the STA returning tothe normal EDCA procedure after a UL MU random access procedure orscheduled UL transmission of a short packet.

In the example illustrated in FIG. 18, the STAs may not know how long towait for a trigger frame to arrive before falling back to the normalEDCA procedure. After the TxOP 1805 finishes, and STA 3 has not receivedits trigger frame, STA3 may fall back to the normal EDCA procedure totransmit its UL data 1810, which had previously been reported to the AP.This may increase the probability that the transmission 1810 collideswith a trigger frame 1815 destined for STA3 using the UL MU randomaccess procedure. It may also create the problem that STA3 may havenothing to send if a trigger frame destined to itself is received aftera successful EDCA access.

In embodiments, a NAV or a prohibitive timer may be provided in theacknowledgement frame responding to the frame carrying the buffer statusreport from the STA. The NAV may be set per STA per access category (AC)and may not apply to STAs that the acknowledgement frame is notaddressed to. After the NAV/prohibitive timer is started in the STA, andbefore expiry of the timer, the per AC EDCA backoff timer may be paused.Alternatively, a timer value may be provided in a broadcast message,such as a beacon frame or other frames providing system information,such as a probe response frame.

The prohibitive timer may be stopped when the STA receives a triggerframe addressed to itself. The timer may be paused when the TxOP holderis the AP or whenever the medium is busy. When the timer expires, theEDCA procedure may be resumed to reduce the possibility of collisionwhen multiple STAs use the same timer value.

In embodiments, the STA may disregard the timer and resume EDCA accessif traffic of a higher priority AC has arrived that has not beenpreviously reported to the AP. Further, the STA may disregard the timerif a certain portion of a DL preamble, such as HE-SIG-B, could not bedecoded.

When responding to the DL traffic, an updated buffer status may bereported together with the acknowledgement, and the prohibitive timermay be restarted. For the same class of traffic, different values of thetimer may apply depending on whether the frame carrying the bufferstatus is acknowledged (such as data) or not (such as anacknowledgement). The pause of EDCA access may not apply after sending abuffer status report, for example, if there is an on-going transmissionattempt due to a previously failed EDCA access. The STA may perform anegotiation of timer values when it associates with the AP.

With UL MU random access, depending on the random access protocol, itmay be possible that some RUs may not be occupied by any STAs, as isdescribed for some of the embodiments above. In this scenario, it mayneed to be determined how to transmit the preamble, especially on theempty RU. Further, the preamble may be designed to enable an accuratestart of packet detection, which may include AGC and time/frequencysynchronization, accurate channel estimation for UL MU PPDU, backwardcompatibility, and unified format for both DL/UL SU/MU transmissions.

In embodiments, such as illustrated in FIG. 7, all STAs may transmit theL-STF, L-LTF, L-SIG, and HE-SIG fields over the entire band. The AP mayassign the information to be carried in the L-SIG and HE-SIG fields inits DL trigger frame. The HE-STF, HE-LTF and HE data fields may betransmitted over the allocated RUs.

FIG. 19 is a diagram of an example UL MU PPDU 1900. In the exampleillustrated in FIG. 19, the HE-STF 1902, HE-LTF 1904 and data 1906fields may be transmitted on the RUs assigned to the user.Alternatively, the HE-STF 1902 and HE-LTF 1904 fields may be transmittedon one or more 20 MHz basic channels on which the data transmission mayoccur. For example, if the AP assigns RUx and RUy to the STA, RUx may beon the first 20 MHz basic channel and RUy on the second 20 MHz channel(assuming the AP may operate on a channel with a bandwidth equal to orgreater than 40 MHz).

In OFDMA, frequency resources presented in the form of sub-channels maybe assigned to different radio links, which may all be in either uplinkor downlink directions. When a signal is transmitted over a sub-channelallocated on one side of the channel relative to the central frequency,it may create interference on the other side of the channel as the imageof the original signal, which may be due to RF I/Q amplitude and phaseimbalances.

FIG. 20 is a graph 2000 of the power spectrum density of a partiallyloaded OFDM signal with RF I/Q imbalance. The graph 2000 in FIG. 20shows a snapshot of a scenario in which, among 256 subcarriers in a 20MHz channel, a sub-channel with subcarriers from 199 to 224 (shown as Asub-channel in the graph) is loaded with data. Due to RF I/Q imbalance,about 23 dB interference is generated in the image of the sub-channelwith subcarriers from −119 to −224 (shown as B sub-channel in thefigure).

In single BSS scenarios, in the OFDMA DL, this interference may not besignificant since the transmit powers on all sub-channels are the sameas are the receive (Rx) powers on these sub-channels at each STA.However, in the OFDMA UL, if there is no power control or the powercontrol is not accurate, the interference at the image sub-channel(e.g., B in FIG. 20) could be significant (e.g., the STA usingsub-channel B is farther away from the AP than the STA using channel A).With MU random access, the STAs may not be able to accurately controlthe transmit power such that received powers at the AP side are aligned.Thus, the interference may be more severe for MU random access.

Accordingly, in embodiments, the trigger frame that allocates RUs forrandom access may be designed such that symmetric random access may beused. This means that the STAs may use the RUs that are symmetricallyallocated around the center frequency, as shown in the diagram 2100 ofFIG. 21.

In scenarios where different STAs have different traffic priorities, theOFDMA random access procedure may be modified to take into account thetraffic priorities of the different STAs. This may allow STAs with thecorrect priorities to access the medium.

In embodiments, when a STA has a frame to send, it may initialize aninternal OFDMA backoff (OBO) to a random value in the range of zero toOFDMA contention window (CWO). For a STA with a non-zero OBO value, itmay decrement its OBO by 1 in every RU assigned to a particular AIDvalue with the TF-R. For a STA, its OBO decrements by the value, unlessOBO=0, equal to the number of RUs assigned to the particular AID valuein a TF-R. OBO for any STA can only be zero once every TF-R. A STA withOBO decremented to zero may randomly select any one of the assigned RUsfor random access and transmit in its frame.

In embodiments, the OFDMA back-off contention window (CWO) may beinitialized to a value based on the traffic type. These may be definedas: Voice: OBO_backoff[AC_VO]; Video: OBO_backoff [AC_VI]; Best effort:OBO_backoff[AC_BE]; and Background: OBO_backoff[AC_BK], whereVoice<Video<best effort<background. This may give different prioritiesto the different traffic types. In other embodiments, specific randomaccess transmit opportunities or random access OFDMA resources may bereserved for specific traffic access categories, and only STAs withtraffic in that access category or higher may be permitted to access themedium during that specific random access opportunity. And in otherembodiments, specific random access OFDMA resources may be reserved forspecific traffic access categories, and only STAs with traffic in thataccess category or higher may be permitted to access the medium duringthat specific random access opportunity. FIGS. 22 and 23 below arediagrams showing the different TxOPs with traffic priorities beingdetermined within each random access opportunity and with trafficpriorities being determined for specific RUs, respectively.

For both cases, the following procedure may be used. The AP may send outa random access trigger to the STAs. The preamble of the random accesstrigger may signal the resources available and the traffic accesscategories allowed. The HE-SIG-B field of the preamble may be used tosignal the information to the STAs, as described above. The commonsection of the SIG-B frame may indicate the resources that are availablefor transmission. This may include the bandwidth of each random accessRU as well as its availability (and in some cases, the AP may desire tosilence or reserve a specific RU).

In one example, the HE-SIG-B field may indicate an allocation of fixedbandwidth RUs only or a mix of different sized RUs. The user specificsection of the HE-SIG-B field may indicate the STAs, groups of STAs andtraffic priorities of the STAs allowed to compete for the correspondingresource or RU. In this case, it may become an RU-specific signalingfield.

On a condition that there are no restrictions, a flag may be placed inthe common HE-SIG-B field to indicate that all STAs may access theresources. In this case, the HE-SIG-B field may be empty. Alternatively,the user specific HE-SIG-B field may be set to a value that indicatesthat all STAs may compete for the resource, for example, the BSSID ofthe BSS. Alternatively, if a group address is not assigned to aresource, it may be inferred that all STAs may access the resource. If atraffic class is not assigned to a resource, it may be inferred that alltraffic classes may access the resource. In a multiple BSSID scenariowith virtual BSSs, the AP may use the common BSSID for the physical APto ensure that all STAs in all BSSs are allowed to compete for theresources.

On a condition that there are restrictions on which STAs are allowed tocompete for the random access resource, a group-ID may be used in placeof the STAID or user specific ID to indicate the group of STAs allowedto compete for the resource. STAs may be grouped based on differentcriterion, such as traffic types, physical location (to minimize hiddennodes), and OBSS interactions (to minimize interference to/from OBSSSTAs). The AP may add a STA to a group by sending a group adding framewith the address of the STA and the address of the group it belongs to.The AP may also remove a STA from a group in the same manner. The STAmay send an ACK to indicate that it has been added to the group. Groupsmay not be mutually exclusive (i.e., a STA may belong to multiplegroups).

On a condition that there are restrictions on the access categories ofthe traffic of the STAs allowed to compete for a resource, the specifictraffic access category (AC) (or minimum traffic class allowed) may besent in the HE-SIG-B field for that specific resource. In the event thatall resources are traffic class restricted, the minimum AC may be sentin the common HE-SIG-B field. In another embodiment, all ACs that arepermitted may be enumerated. Table 1 below shows a possible SIG-Bstructure for traffic and group restrictions.

TABLE 1 SIG-B Structure Common [RU1 RU2 RU3 RU4 RU5 26 tone RU RU6 RU7RU8 RU9] allocation User specific/RU specific RU1: Grp 1, AC_VO Allowsgroup 1 with at least voice traffic RU2: Grp 2, AC_BE Allows Group 2with at least best effort traffic (i.e. all traffic) RU3: AC_VI Allusers, at least video traffic RU3: AC_BE All users, all traffic RU3:AC_BE All users, all traffic RU3: AC_BE All users, all traffic RU3:AC_BE All users, all traffic RU3: AC_BE All users, all traffic RU3:AC_BE All users, all traffic

FIG. 22 is a signal diagram 2200 of an example random access continuousTxOP with traffic priorities determined within each random accessopportunity. In the example illustrated in FIG. 22, the AP 2202transmits a first random access trigger 2212 that specifies a minimum ofAC_VO. In response to the trigger 2212, STA1 2204 and STA 3 2206transmit AC-VO frames of 2214 and 2216, respectively. The AP 2202transmits a second random access trigger 2218 that specifies a minimumof AC_VI. In response to the trigger, STA 1 2204 and STA2 2206 transmitAC_VI frames 2220 and 2222, respectively, and STA4 2208 transmits anAC_VO frame 2224. The AP 2202 transmits a third random access trigger2226 that specifies a minimum of AC_BE. In response to the trigger 2226,STA1 2204 transmits an AC_VI frame 2228, and STA2 2208 and STA4 2210transmit AC_BE frames 2230 and 2232, respectively. The AP 2202 may thensend a multi-STA block ACK 2234 for all of the transmissions in thecontinuous random access TxOP.

FIG. 23 is a signal diagram 2300 of an example random access continuousTxOP with traffic priorities determined for specific RUs. In the exampleillustrated in FIG. 23, an AP 2302 transmits a first random accesstrigger 2312 specifying a minimum of AC_VO on RU1 2314 and a minimum ofAC_BK on RU2 2316. In response to the first random access trigger 2312,STA1 2304 acquires RU1 and transmits an AC_VO frame 2318 on RU1, andSTA3 2308 acquires RU2 and transmits an AC_BE frame 2320 on RU2. The AP2302 transmits a second random access trigger 2322 specifying a minimumof AC_VO on RU1 and a minimum of AC_BK on RU2. In response to the secondtrigger frame 2322, STA2 2306 acquires RU1 and transmits an AC_VO frame2328, and STA4 2310 acquires RU2 and transmits an AC_VI frame 2330. TheAP 2302 may then send a multi-STA block ACK 2332.

Although features and elements are described above in particularcombinations, one of ordinary skill in the art will appreciate that eachfeature or element can be used alone or in any combination with theother features and elements. In addition, the methods described hereinmay be implemented in a computer program, software, or firmwareincorporated in a computer-readable medium for execution by a computeror processor. Examples of computer-readable media include electronicsignals (transmitted over wired or wireless connections) andcomputer-readable storage media. Examples of computer-readable storagemedia include, but are not limited to, a read only memory (ROM), arandom access memory (RAM), a register, cache memory, semiconductormemory devices, magnetic media such as internal hard disks and removabledisks, magneto-optical media, and optical media such as CD-ROM disks,and digital versatile disks (DVDs). A processor in association withsoftware may be used to implement a radio frequency transceiver for usein a WTRU, UE, terminal, base station, RNC, or any host computer.

1. A wireless transmit/receive unit (WTRU) comprising: a receiver and aprocessor, the receiver and the processor being configured to detect atrigger frame for uplink (UL) multi-user (MU) transmission, the triggerframe including an assignment of resource units (RUs) for random accessin upcoming UL MU physical layer protocol data units (PPDUs) and anindication that the trigger frame is one of a plurality of triggerframes in a cascading sequence of trigger frames in an MU transmissionopportunity (TxOP); and a transmitter, the processor and transmitterbeing configured to select one of the RUs in the assignment of RUs for arandom access transmission and send the random access transmission onthe selected one of the RUs.
 2. The WTRU of claim 1, wherein thereceiver is further configured to receive an acknowledgement for therandom access transmission, the acknowledgement being aggregated withone of the plurality of trigger frames in the sequence.
 3. The WTRU ofclaim 1, wherein the processor is further configured to enter a sleepstate in response to a last trigger indication in at least one triggerframe of the plurality of trigger frames in the sequence.
 4. The WTRU ofclaim 1, wherein the trigger frame includes a field that polls for atraffic buffer status of the WTRU.
 5. The WTRU of claim 1, wherein theassignment of RUs for random access in the upcoming UL OFDMAtransmission is for an integer number, M, of RUs, and the processor andthe transmitter are configured to select the one of the RUs in theassignment of RUs for the random access transmission by: generating arandom backoff index, R, from a range of integer values between zero anda defined maximum value, on a condition that R>M, holding the randomaccess transmission and re-setting the value of R to R=R−M, and usingthe re-set value of R to compete for the next MU random accessopportunity in the TxOP.
 6. The WTRU of claim 5, wherein the processorand the transmitter are further configured to select the one of the RUsin the assignment of RUs for the random transmission by transmitting onan Rth RU of the RUs included in the assignment of RUs for random accesson a condition that R≤M.
 7. The WTRU of claim 1, wherein the triggerframe includes a common information field that carries information toset a high efficiency (HE)-SIG-A field in the upcoming UL MU PPDU.
 8. Amethod, implemented in a wireless transmit/receive unit (WTRU), themethod comprising: detecting a trigger frame for uplink (UL) multi-user(MU) transmission, the trigger frame including an assignment of resourceunits (RUs) for random access in upcoming UL MU physical layer protocoldata units (PPDUs) and an indication that the trigger frame is one of aplurality of trigger frames in a cascading sequence of trigger frames inan MU transmission opportunity (TxOP); selecting one of the RUs in theassignment of RUs for a random access transmission; and sending therandom access transmission on the selected one of the RUs.
 9. The methodof claim 8, further comprising receiving an acknowledgement for therandom access transmission, the acknowledgement being aggregated withone of the plurality of trigger frames in the sequence.
 10. The methodof claim 8, further comprising entering a sleep state in response to alast trigger indication in at least one trigger frame of the pluralityof trigger frames in the sequence.
 11. The method of claim 8, whereinthe trigger frame includes a field that polls for a traffic bufferstatus of the WTRU.
 12. The method of claim 8, wherein the assignment ofRUs for random access in the upcoming UL OFDMA transmission is for aninteger number, M, of RUs, and the selecting one of the RUs in theassignment of RUs for a random access transmission comprises: generatinga random backoff index, R, from a range of integer values between zeroand a defined maximum value, on a condition that R>M, holding the randomaccess transmission and re-setting the value of R to R=R−M, and usingthe re-set value of R to compete for the next MU random accessopportunity in the TxOP.
 13. The method of claim 12, further comprisingtransmitting on an Rth RU of the RUs included in the assignment of RUsfor random access on a condition that R≤M.
 14. The method of claim 8,wherein the trigger frame includes a common information field thatcarries information to set a high efficiency (HE)-SIG-A field in theupcoming UL MU PPDU.
 15. An access point (AP) comprising: a processorand a transmitter, the processor and the transmitter being configured togenerate and transmit a trigger frame for uplink (UL) multi-user (MU)transmission to a plurality of stations (STAs), the trigger frameincluding an assignment of resource units (RUs) for random access inupcoming UL trigger-based physical layer protocol data units (PPDUs) andan indication that the trigger frame is one of a plurality of triggerframes in a cascading sequence of trigger frames in an MU transmissionopportunity (TxOP); and a receiver, the receiver and the processor beingconfigured to receive the UL trigger-based PPDUs, the receivedtrigger-based PPDUs including at least one frame from at least one ofthe plurality of STAs on one of the assigned RUs, wherein thetransmitter and the processor are further configured to transmit anacknowledgement for the at least one frame, the acknowledgement beingaggregated with one of the plurality of trigger frames in the sequence.16. The AP of claim 15, wherein at least one frame of the plurality oftrigger frames in the sequence includes a last trigger indication thattriggers STAs to enter a sleep state.
 17. The AP of claim 15, whereinthe trigger frame includes a field that polls for a traffic bufferstatus of STAs.
 18. The AP of claim 15, wherein the trigger frameincludes a common information field that carries information to set ahigh efficiency (HE)-SIG-A field in the UL MU PPDU.